The aims of the proposed project are to better define the cardiovascular responses that occur during isometric exercise and to investigate their controlling mechanisms. This study involves investigations of both the initiation of cardiovascular responses and the manner by which they are mediated. Two experimental models will be used in the proposed study. An experimmntal model has been developed which utilizes a volitional effort to induce cardiovascular adaptation to isometric exercise. This program consists of operantly conditioning cats to lift increasing amounts of weight and to hold the load for a preset time delay. The initial objective is the functional characterization of the cardiovascular system that is adapting to isometric exercise in chronically instrumented awake animals. The cardiovascular responses are thought to arise either reflexly from the exercising muscle or from a direct action of the motor cortex on the cardiovascular "center". Measurements will be made of cardiovascular function of the exercising animal both before and after deafferentation of the exercising forelimb. Cardiovascular measurements will also be made in the exercising animals both before and after unilateral ablation of the motor cortex. Experiments to be performed on acutely prepared anesthetized animals are designed to identify and characterize the muscle afferents that are involved in the cardiovascular reflex, and to localize and examine the muscle receptors. In addition, the spinal aspect of the pressor reflex will be investigated. Analysis of the data obtained from this study should greatly enhance our understanding of the mechanism involved in the cardiovascular responses to isometric exercise. It is important to better understand the cardiovascular responses to this form of activity since it is performed innumerable times daily by normal individuals and patients with heart disease and is used as a stress test for the cardiovascular system.